The above-identified definition includes in particular devices that monitor cardiac activity and generate impulses of stimulation, resynchronization, defibrillation, and/or cardioversion in the event the device detects a disorder in heart rate. It also includes, for example, neurological devices, pumps for distribution of medical substances, cochlear implants, and implanted biological sensors, as well as devices for measurement of pH or bio-impedance (such as trans-pulmonary impedance or intracardiac impedance measurements).
With such devices, it is possible to operate a data exchange with a “programmer,” which is an external instrument that can be used to check the parameter settings of the devices, to read information recorded by the devices, to register information with the devices, and to update the internal control software of the devices. This data exchange is carried out by telemetry, i.e., by a technique of remote transmission of information, without galvanic contact. Until now, telemetry has primarily been carried out by magnetic coupling between coils in the implanted device and the programmer, which is a technique known as “process by induction.” This technique has certain disadvantages, however, because of the low range of an inductive coupling, which necessitates placing a “telemetry head” containing a coil in the vicinity of the implantation site of the active implantable medical device.
Implementation of a different nongalvanic coupling technique has been proposed, using the two components of an electromagnetic wave produced by emitting/receiving circuits operating in the field of radio frequencies (RF), typically at frequencies around a few hundred MHz. This technique, known as RF telemetry, makes it possible to program or interrogate implants at distances greater than 3 meters, and thus carry out information exchanges without having to use a telemetry head, and even without intervention of an external operator. U.S. Patent Application Publication Nos. US2003/0114897 and US2003/0149459 describe implants and programmers equipped with such RF telemetry circuits. These RF circuits require, however, a current supply that is greater than what is necessary for the other circuits of the implant (e.g., the stimulation and detection circuits). For example, the current consumption of an RF circuit can exceed 3 mA during emission phases.
In the case of defibrillators, taking into account the significant amount of current required by circuits used to apply shock therapy, the batteries used have low internal resistance and can supply without difficulty currents of about a few mA. On the other hand, pacemakers and similar devices, such as multisite or resynchronization devices, are generally supplied by small-size lithium-iodine batteries (or their equivalent), taking into account the low operating current required by the stimulation and detection circuits. These batteries have an internal resistance of about 100 Ω at the beginning of their life, which can increase to 1 kΩ, 2 kΩ, or more as the battery discharges. This internal resistance is not a problem for circuits with low consumption, but can prevent one from being able to provide RF circuits with the required level of current.
A first solution is to use a different type of battery, for example, a reduced size lithium-manganese (LiMnO2) weldable button battery with low impedance. There are such batteries whose characteristics are: diameter 12.2 mm, height 1.4 mm, capacity 27 mA/h, nominal voltage 3 V, self-discharge maximum 1% per annum, and which can provide currents of several mA. The current of RF circuits is exclusively provided by the button battery. When the button battery can no longer provide the current, the lithium battery of the pacemaker can provide a low current of 10 μA, which allows the transmitter-receiver to work in pulsated mode. The peak current is provided by a capacitor belonging to a supply circuit controlling the voltage of the battery.